How to set the easing of keyframe with Python (fast, slow, etc.)
To select the keyframe points with frame = time of the scale fcurves, and set easing to 'EASE_IN'

import bpy

context = bpy.context
obj = context.object

action = obj.animation_data.action
fcurves = [fc for fc in action.fcurves if fc.data_path == "scale"]
#fcurve = action.fcurves.find("scale", index=0) # location.x fcurve
time = 1 # frame
for fcurve in fcurves:
    # iterate thru keyframe Points and change easing of those at frame = time
    for kfp in fcurve.keyframe_points:
        # ('AUTO', 'EASE_IN', 'EASE_OUT', 'EASE_IN_OUT')
        if kfp.co.x == time:
            print("scale.%s easing set to EASE_IN at frame %d" % ("xyz"[fcurve.array_index], time))
            kfp.easing = 'EASE_IN' #  auto 


alternatively can create an action, fcurve and keyframe points eg make a new action with scale fcurves and add one keyframe (40, 3.9) with easing 'AUTO'.

import bpy
context = bpy.context
# create a new action and assign it to object
action = bpy.data.actions.new("ScaleAction")
context.object.animation_data.action = action
# fcurve data_path
data_path = "scale"
# (frame, value) for keyframe point
time = 40
value = 3.9
for axis in [0, 1, 2]:
    # new fcurve
    fc = action.fcurves.new(data_path, index=axis)
    # add a new keyframe point
    fc.keyframe_points.add(count=1)
    for kfp in fc.keyframe_points:
        kfp.co = (time, value)
        kfp.easing = 'AUTO'